Reverse process of coprecipitation of dissolved organic carbon with Fe(III) precipitates in a lake

Reverse process of the coprecipitation of dissolved organic matter with Fe(III) precipitates in a lake is reported. Water containing a slight amount of dissolved oxygen from the hypolimnion of Lake Onogawa was sealed in glass bottles, and some changes in the constituents with time were followed. The water sample contained 0.1 mg l⁻¹ dissolved oxygen at the beginning of the experiment, which decreased to 0.0 mg l⁻¹ within 24 h. In response to the depletion of dissolved oxygen, there were increases in dissolved Fe from 8.4 to 11.4 mg l⁻¹ and dissolved organic carbon (DOC) from 5.1 to 6.9 mg l⁻¹ after 72 h. At the beginning of the experiment, more than 2 mg l⁻¹ of insoluble Fe, which was thought to be Fe(III) precipitates, existed in the water samples. When the water samples became anoxic, the preexisted Fe(III) must have been reduced to soluble Fe(II) over time, resulting in the increase of dissolved Fe. Simultaneous with Fe(III) reduction, coprecipitated DOC with Fe(III) must have been released. The reverse process of the coprecipitation of dissolved organic matter with Fe(III) precipitates found in the present study strongly supports in situ coprecipitation of dissolved organic matter with Fe(III) precipitates.